The present invention relates to an apparatus for mixing two or more solute-containing solutions into a uniform solution system. The invention also relates to an apparatus in which two or more solute-containing solutions are mixed uniformly to cause a chemical reaction to occur, yielding a reaction product. The invention further relates to an apparatus for producing silver halide (hereinafter abbreviated as "AgX") emulsion grains which are useful in the photographic industry. The invention also relates to an apparatus for chemically sensitizing said AgX emulsion grains. The invention further relates to an apparatus for adding solutions of photographically useful additives to an AgX emulsion. The invention also relates to an apparatus for producing AgX emulsions, having a capacity of at least 100 liters.
Mixing two or more solute-containing solutions into a uniform solution system is an important basic operation that is commonly employed in chemistry. Conventionally, this operation is performed by rotating agitating blades or a magnetic stirrer; these apparatus may be used either independently or in combination with baffle plates, a jet mixer, shaking or ultrasonic agitation.
Whether two or more solute-containing solutions can be efficiently mixed into a completely uniform system often determines if the reaction involved will proceed smoothly; in addition, it can also influence the yields of the reaction product and by-products. Consider, for example, a reaction between two liquid phases. This chemical reaction occurs under the least favorable conditions for uniform mixing with stirring. In such a reaction, solutes A and B dissolved in the two liquid phases contact only at their interface to enter into the reaction. Since the resulting reaction product L is not eliminated from the interface, a high concentration of the reaction product is produced in the neighborhood of the interface. As a result, the intended reaction A+B.fwdarw.L no longer takes place. Instead, undesired side reactions L+A.fwdarw.LA and L+B.fwdarw.LB occur, increasing the yields of the by-products. If the procedure of mixing two liquid phases is not effectively performed to produce a uniform solution system, the reaction rate is generally reduced; and, at the same time, the yield of the main reaction product decreases whereas the yields of unwanted by-product increases.
Therefore, in a generalized chemical reaction for producing products L, M, N, . . . from reactants A, B, C, . . . in solution, the added reactants must be rapidly agitated to create a uniform mixture system. This is particularly important if the chemical reaction concerned is irreversible. In general, the chemical reaction rate is generally proportional to the concentrations of reactants; in addition, the higher the temperature, the faster the reaction rate. Hence, the reaction rate is high in the region of high solute concentration near the inlets where the solute-containing solutions are added. The heat generation accompanying the reaction will further increase the reaction rate, and cause the reaction to proceed at increased rates in certain areas. In a reversible reaction system, such an imbalance in the reaction is effectively corrected since it proceeds in opposite direction after a uniform mixture forms. However, in an irreversible reaction system, the rate of reverse reaction which occurs after the formation of a uniform mixture is so slow that the non-uniform reaction product that results from the local high-speed reaction will often remain unremoved.
Consider, for example, a reaction for producing AgX emulsion grains from a silver salt and a halide salt (which is hereinafter abbreviated as an "X.sup.- salt". When a silver salt and/or X.sup.- salt solution is added to the solution in a reaction vessel (which is hereinafter called the "vessel solution"), the concentration of the added solution is very high in the neighborhood of the inlet through which it is added. It usually takes a considerable time for the added solution to be uniformly mixed with the entire vessel solution, and, as a result, the following various disadvantages occur:
(1) A silver halide, for example, AgBr in aqueous solution, has solubility product of 1.58.times.10.sup.-11 mole.sup.2 /l.sup.2 at 60.degree. C. However, the aqueous solutions of AgNO.sub.3 and NaBr which are added to the vessel solution usually have concentrations in the range of 10.sup.-1 to 3 moles/l, so the degree of supersaturation compared to the equilibrium concentrations of the solutes in solution is as high as 10.sup.4 -10.sup.6 in the neighborhood of the inlets for their addition. As a result, the reaction represented by the formula AgNO.sub.3 +KBr.fwdarw.AgBr+KNO.sub.3 will proceed at an extremely high rate around those inlets. The rate of this reaction is proportional to the solute concentrations which in turn are dependent on the degree of agitation; therefore, the rate of the reaction is largely dependent on the degree of agitation of the vessel solution.
Stated more specifically, with reference to the stage of nucleation, the number or size of the nuclei formed and the occurrence of twinned faces and defects such as dislocations will depend on the degree of supersaturation in the vessel solution. Therefor, the characteristics of those nuclei will largely depend on the degree to which the vessel solution is agitated. In other words, the size and shape of the resulting AgX grains will largely depend on the degree of agitation in the vessel solution. This dependency deteriorates the consistency of AgX emulsion produced with small equipment at the laboratory scale and even more with plant facilities for large-scale production on a commercial basis. This problem is particularly great in the production of monodispersed parallel double twinned grains described in Japanese Patent Application No. 315741/1988; in such a process it is important to adjust the degree of supersaturation during nucleation.
(2) If, during the growth of mixed crystals consisting of at least two of AgCl, AgBr and AgI, unevenly mixed solutions of X.sup.- salt and silver salt undergo a non-uniform reaction in the neighborhood of the inlets through which the solutes are added, heterogeneous mixed crystals will form. The contents of Cl.sup.-, Br.sup.- and I.sup.- will not be uniform either in the grains or between the grains.
(3) In the case of light-sensitive AgX grains, the reduction sensitized silver nuclei that form in those grains during their growth contribute to higher sensitivity; but, on the other hand, they cause nonuniform reduction sensitization. The formation of reduced silver nuclei on AgX grains in the same vessel solution is accelerated in areas having high Ag.sup.+ concentrations, so that the non-uniform formation of reduced silver nuclei is promoted in the neighborhood of the inlet through which AgNO.sub.3 is added.
In the three cases described above, the non-uniform reaction, once it occurs, causes the AgX grains to keep growing, so the product of such non-uniform reaction will remain immobilized in the AgX grains under the usual temperature conditions (.ltoreq.100.degree. C.).
Another case that requires two or more solutions to be mixed rapidly and uniformly is the chemical sensitization of AgX emulsions. The process of chemically sensitizing AgX emulsions usually involves adding 10.sup.-5 to 10.sup.-7 moles of chemical sensitizers to one mole of AgX emulsion heated at 40.degree.-80.degree. C. and performing chemical ripening for a period of from about 5 to 50 minutes. In this case, silver sulfide and/or gold sulfide forms rapidly in areas where the chemical sensitizers are added, causing chemical sensitization nuclei to form unevenly between AgX grains. As a result, disadvantages occur: first, the photographic sensitivity differs between individual grains, so that no contrasty image can be obtained; second, the desired interimage effect cannot be obtained in such a multi-layered system. It is therefore necessary to develop an apparatus that allows two solutions to be mixed uniformly and rapidly.
Uniform mixing is also required when adding photographically useful additives to AgX emulsions. In such reactions the additives undergo an irreversible reaction with the AgX emulsion grains (particularly in the case of reaction with foggants or reduction sensitizers).
To overcome the aforementioned disadvantages, various efforts have been made to modify the methods for adding solutions of reactants (which are hereinafter sometimes referred to as "solute solutions") or mixing them with vessel solutions under agitation. In early versions of the apparatus for producing AgX emulsion grains, solutions of reactants were added through tubes to form cascades that fell on the surface of the vessel solution. This method, however, was not very effective since the area near the surface of the vessel solution was not usually as thoroughly agitated as the other areas. When agitating blades were installed near the surface of the vessel solution to mix the added solute solutions under rapid agitation, extensive foaming occurred which deteriorated, rather than improved, the efficiency of agitation. The resulting foam, although being part of the vessel solution, was hardly mixed under agitation. The method was also disadvantageous because the solute solutions, when added in cascade form, also caused foaming in the vessel solution. Attempts were made to improve the efficiency of agitation by injecting the solute solutions into areas near the agitating blades in the vessel solution (directly adding them into the vessel solution) through addition tubes having outlets located near the agitating blades in the vessel solution. For details of these improvements, reference may be had to the description in JP-B-55-10545 (the term "JP-B" as used herein means an "examined Japanese patent publication"), U.S. Pat. Nos. 3,785,777, 3,790,386, 3,692,283 and 3,415,650.
The methods described in the above patents improve efficiency, but they still suffer from the same disadvantages as the previous methods: concentrated laminar flows of the reactant solutions flowing out of the addition tubes cause a non-uniform reaction in the vessel solution, and it takes considerable time for the added solutions to be uniformly mixed with the vessel solution. Further, in commercial production at plants where larger diameter addition tubes are employed, the added solute solutions flow out as laminar flows having greater cross sections, which also causes a non-uniform reaction in the vessel solution. In addition to the above-described contrivances, those directed to modifying the shape of agitating blades and the positions where they are installed, as well as the use of baffle plates as agitating means have been proposed and are well known in the art.
Each of the attempts previously made to solve the aforementioned problems of the prior art is based on analyses of macroscopic phenomena that accompany agitation (e.g., the state of liquid flows, the size and magnitude of turbulence, the quantity of flows delivered, and power consumption); none of them have resulted from studies at the atomic or molecular level. For example, Buche proposed in 1938 that, to proportionally enlarge an agitation tank while holding the velocity constants for dissolution, extraction, heat transfer, etc. in the reaction vessel constant, "the enlargement be effected in such a way that the power consumption per unit volume of the solution is the same". In 1951-1952, Rushton made a proposal, developing this conclusion, but it was not directly related to the idea of agitation for producing a uniform mixture at the atomic or molecular level. In fact, the above-described problems with the process of uniform mixing under agitation still remain unsolved even today. For more detailed discussions of these problems, reference may be had to "Kagaku Kogaku Binran (Handbook of Chemical Engineering", ed. by The Society of Chemical Engineers, Japan, Chapter 20, Maruzen ( 1988), "Kongo oyobi Kakuhan (Mixing and Agitation)", edited by T. Misawa, Kagaku Kogyosha (1989), and "Shinjikkenkagaku Koza (A New Course in Experimental Chemistry)1", Kihon Sosa (Basic Operations)II, Section 5-2, Maruzen (1975).
If one attempts the uniform mixing process described above by merely relying upon agitating blades, vigorous agitation is necessary. This can damage AgX grains and produce AgX emulsions that will experience extensive fogging during development.